To produce oil or gas from a reservoir, a well is drilled into a subterranean formation, which may be the reservoir or may be adjacent to the reservoir. A subterranean well includes a wellhead and at least one wellbore from the wellhead penetrating the earth. Typically, a wellbore must be drilled thousands of feet into the earth. Generally, as the depth of the formation increases, the static pressure and temperature of the formation increases.
Subterranean wells (e.g., hydrocarbon producing wells, water producing wells, or injection wells) are often stimulated by hydraulic fracturing treatments. In traditional hydraulic fracturing treatments, a treatment fluid, which may also function simultaneously, or subsequently as a carrier fluid, is pumped into a portion of a subterranean formation at a rate and pressure sufficient to break down the formation and create one or more fractures therein. Typically, particulate solids are suspended in a portion of the treatment fluid and then deposited into the fractures. These particulate solids, or “proppant particulates,” serve to prevent the fractures from fully closing once the hydraulic pressure is removed. By keeping the fractures from fully closing, the proppant particulates aid in forming conductive paths through which fluids produced from the formation may flow.
The degree of success of a fracturing operation depends, at least in part, upon fracture porosity and conductivity once the fracturing operation is complete and production has begun. Traditional fracturing operations place a large volume of proppant particulates into a fracture to form a “proppant pack” in order to ensure that the fracture does not close completely upon removing the hydraulic pressure. The ability of proppant particulates to maintain an open fracture open depends upon the ability of the proppant particulates to withstand fracture closure pressure and, therefore, is typically proportional to the volume of proppant particulates placed in the fracture. The porosity of a proppant pack within a fracture is related to the interconnected interstitial spaces between abutting proppant particulates. Thus, the fracture porosity is closely related to the strength of the placed proppant particulates and often tight proppant packs are unable to produce highly conductive channels within a fracture, while reducing the volume of the proppant particulates is unable to withstand fracture closures.
One way proposed to combat the problems inherent in tight proppant packs involves the use of proppant pillars. As used herein, the term “proppant pillar” refers to a coherent body of consolidated proppant particulates that generally remain a coherent body. Proppant pillars are comprised of a plurality of proppant particulates formed into a tight cluster and are capable of withstanding fracture closure pressures. The use of proppant pillars, therefore, may reduce the likelihood of partial or complete fracture closure. The proppant pillars placed into a fracture do not abut together perfectly and therefore may achieve infinite conductivity channels (e.g., unobstructed pathways) for produced fluid flow.
While proppant pillars can overcome certain issues associated with tight proppant packs, in practice several issues may prevent their optimal performance. For example, proppant pillars of low-strength proppant, such as silica sand, are not conventionally used in high closure stress fractures, or a fracture having a closure pressure exceeding about 8,000 psi, due to their inability to withstand such pressure. Thus, alternative proppants of higher strength are usually employed in high closure stress fractures. Unfortunately, high strength proppants typically cost more than low strength proppants; thus, increasing costs associated with the hydraulic fracturing of high closure stress fractures.
In addition, although proppant pillars provide greater conductivity channels for fluid flow, the problems associated with formation and migration still persist. Fines are formed in a number of ways. For example, fines are generated when proppant particulates are crushed due to their inability to withstand fracture closure stress. Fines are also formed due to mechanical and/or natural degradation of subterranean formations, especially in soft rock reservoirs like, sandstone, shale or coal. Regardless of how the fines are formed, fine migration decreases well performance even in fractures having proppant pillars.
Accordingly, there is a need for improved methods of propping a fracture using low-strength proppant particulates in order to reduce costs while maintaining the minimization of fine migration especially in soft rock reservoirs.